SKY REPORTER: July 2012

Sky Data

Sunday July first, sunrise is at 5:28 a.m. Eastern Daylight Time. That evening the Sun, located in the constellation Gemini, sets at 8:31 p.m.

On Sunday the 15th sunrise is at 5:38 a.m. and sunset occurs that day at 8:25 p.m.

The last day of the month, Tuesday July 31, the Sun is in the constellation of the Crab, rising at 5:52 a.m. and setting at 8:12 that evening. During July the length of time the solar disk is above the horizon each day decreases by 42 minutes and its altitude at noon, the daily maximum, drops from 72 degrees of arc on the first to 67 degrees on the 31st.

During this month average overnight temperatures recorded in Central Park increase from 67 degrees Fahrenheit on the first to 69 degrees on the last day of July. Despite decreasing daily minutes of sunlight and lower solar angles this month, land and water in our area retain sufficient heat so average temperatures recorded in Central Park reach an annual peak around the third week of July.

Principal Phases of the Moon are:

Full Moon

July 3

Last Quarter

July 10

New Moon

July 19

First Quarter

July 26

Sky Cast

The evening of July first Mercury, Mars, Saturn, and the bright gibbous Moon are all above the horizon at sunset. At 9:59 p.m. Mercury, in the constellation of the Crab, is first of this group to set. Neptune is now in Aquarius and rises at 11:09 p.m. Mars, in Virgo, sets at12:16 early on the morning of Monday July 2nd. Uranus rises at 12:37 a.m. between stars of Pisces and Cetus. Saturn, always a beautiful telescopic sight, is 24 degrees of arc east of Mars in Virgo and sets at 1:22 a.m. Jupiter rises at 3:06, followed 28 minutes later by Venus. The Moon sets at 4:13 the morning of July second.

On the Fourth of July Earth reaches the point in its elliptical orbit known as aphelion when we are furthest from the Sun at a distance of about 94,508,000 miles.

At five a.m. the following morning, as the sky begins to brighten with civil twilight, brilliant Venus is 16 degrees above the eastern horizon, less than two degrees above first magnitude star Aldebaran, and five degrees below Jupiter.

As twilight fades Sunday July 15th elusive Mercury, now rapidly moving closer toward the Sun’s direction, sets at 9:07 p.m. At that time Mars and Saturn are both about 30 degrees of arc above the southwestern horizon.

At 10 p.m. that evening the great globular star cluster in Hercules reaches the meridian. This 6th magnitude object is a magnificent sight with telescopes large enough to resolve the cluster into individual stars. It is 25,000 light years from us, extends over an area of sky about the size of the Full Moon’s disk, and has an age over 10 billion years. This and other globular clusters contain some of the oldest stars in the Galaxy.

Later on the night of July 15, Mars sets at 11:37 p.m. Soon after midnight the Red Planet is followed below the horizon by Saturn at 12:27 a.m. Jupiter rises at 2:21, and Venus arrives in the eastern sky at 3:00 a.m. At 4:00 a.m. that morning the thin waning crescent Moon is about six degrees above the horizon. Then for about an hour until morning twilight intervenes Jupiter, Venus, and Aldebaran form a small triangle low in the eastern sky to the upper right of the crescent Moon, with about 8% of its disk illuminated.

During July nights, when looking skyward from locations far from urban lights, the Band of the Milky Way may be seen extending across the entire sky from Perseus in the northeast to Scorpius in the south. This edgewise view of the Galaxy’s disk is highlighted by several striking asterisms including stars such as Vega, Altair, and Deneb in the Summer Triangle; the hook shaped chain of stars punctuated by Antares in Scorpius; and the Teapot asterism of Sagittarius.

Vega is a white star in the constellation Lyra, and has a distance of 25 light years from the solar system. With an apparent magnitude of 0.03 it is the brightest star in the Summer Triangle and long served as a standard for the magnitude system that compares relative brightness of celestial objects. Larger positive magnitude numbers indicate lower rankings of brightness. Each magnitude unit indicates a brightness difference of about 2.5 times. Vega is a main sequence star and produces light and heat by the fusion of hydrogen to helium in its core.

Altair with magnitude 0.77 is the brightest star in Aquila and marks the southern corner of the Summer Triangle. It is one of the closest bright stars to us, with a distance just under 17 light years. As with Vega, Altair is a white main sequence object in the prime of its existence.

At a distance of about 3200 light years Deneb is one of the most distant individual stars visible to unaided eyes. It is a white supergiant with apparent magnitude 1.25. It marks both the tail of Cygnus the Swan and the top of the Northern Cross asterism. About 7,000 years ago Deneb was Earth’s north polar star.

Antares, brightest star in Scorpius, has an apparent magnitude of 1.06 and is a red supergiant in the last stages of its evolution. Due to exceptional mass it was the first star of hundreds in the Scorpius Centaurus stellar association to have evolved into the red supergiant stage.

The evening of Tuesday July 24, the wide waxing crescent Moon is in Virgo about five degrees south of Mars. One day later the Moon, still in Virgo, moves eastward to a position about four degrees south of Spica and eight degrees below Saturn.

On the morning of July 31, Jupiter rises at 1:32 a.m. followed about an hour later by Venus at 2:39. Mercury is invisible against the brilliance of dawn when it rises at 5:45 a.m., just 7 minutes before the Sun. On the last evening of July Mars sets at 10:55 p.m. and Saturn drops below the horizon at 11:26 p.m.

Sky Notes

Artist’s conception of Curiosity working at Gale crater.

Credit: NASA/JPL-Caltech

On June 5th we welcomed about 700 visitors to the Cullman Hall of the Universe under the great Hayden Sphere to witness the transit of Venus via NASA video. It was a pleasure to have an opportunity to meet many of you and discuss that rare celestial phenomenon. In addition to its visual and historical apeal the recent transit provided planetary researchers with important information. Although composition of the Venusian atmosphere is already quite well known thanks to decades of close study via orbiters and landers, comparisons of such data with telescopic analyses made during the recent transit are expected to help researchers refine similar methods used to study the vastly more remote atmospheres of extra-solar planets.

In light of progress made during the past century, it is difficult to even imagine what new wonders will be available on the evening of December 10, 2117 when many will pause from swirls of holiday activity to consider Venus as it next crosses the face of the Sun.

Recent media attention was focused on Voyager 1, a durable spacecraft launched 35 years ago that produced many high resolution images of Jupiter and Saturn, then continued its travels much further from the Sun. The venerable craft is still sending reports to Earth from its current distance of 119 astronomical units. This is farther than any object constructed by humans has ever traveled from Earth. By comparison, at their closest approaches, Venus and Mars are only about a third of an A.U. from our world. An astronomical unit is equal to the average distance between Earth and the Sun. They were first used centuries ago when only relative distances of planets were known. In modern times, when precise distances are available, rather than dealing with huge numbers such as billions of miles when describing extreme distances in the Solar System, A.U.’s provide easier visualization of spatial comparisons.

Smaller Revised Target Oval for Mars Rover Curiosity at Gale Crater.

Credit: NASA/JPL

NASA researchers reported that Voyager 1 is now in a zone where the intensity of cosmic rays arriving at the craft from interstellar space is rapidly increasing, apparently the result of diminished opposing outward pressure from charged particles in the solar wind.

If you would like look on a clear night and point toward Voyager 1’s current direction in the sky, it’s about halfway between Vega and Antares, one degree east of the fifth magnitude double star 60 Herculis. Voyager 2 is 98 A.U. from us, but it’s toward the southern constellation Telescopium, always beneath New York’s horizon.

While both Voyager 1 and 2 are enduring legacies from an early era of planetary exploration, a new chapter is due to open next month with arrival of the Mars Science Laboratory and its rover Curiosity at the Red Planet’s Gale crater early on the morning of August 5th. The mission’s principal aim will be to search the Martian surface for evidence the planet once had an environment able to support microbial life.